Of the compounds now recommended to perform such oxidation there can be mentioned alkaline peroxides, hydrogen peroxide and alkaline bichromates. Use of alkaline peroxides or hydrogen peroxide, however, often cause certain overoxidation phenomena to occur particularly in certain dyes belonging to the class of sulfur dyes. Use of bichromates often leads to a notable fading of the shades; further, it involves a pollution of the effluent because of the presence of chrominum salts in the waste waters.
Therefore, it has been proposed to use, as oxidation agents, alkaline metals chlorites and especially sodium chlorite whose use does not give rise to any phenomenon of overoxidation or dulling of the shades. However, it is known that the use of sodium chlorite as an oxidation agent requires the use of this compound at a relatively alkaline pH range; actually, it is imperative in this application to avoid any decomposition of the chlorite solution, which decomposition can occur very rapidly when working in an acid pH zone, at the usual application temperatures, generally close to or greater than 50.degree. C. The chlorine or chlorine compounds that result from the decomposition of alkaline chlorites run the risk of deeply modifying the shades of the dyes applied, and particularly of sulfur dyes.
It is known, on the other hand, that respecting the alkaline conditions inevitably leads to a reduction of the effectiveness of the oxidizing treatment because of the relatively much smaller oxidizing power of the chlorite in an alkaline medium. It is necessary then to perform the oxidation treatments at high temperatures, often greater than 80.degree. C., also at the same time maintaining oxidation periods which are necessarily too long.